1. Field of the Invention
This invention relates to wafer handling apparatus and a method for manufacturing same. Thin wafers such as semiconductor masters are essentially circular and require coaxial alignment in wafer-carrying "boats". More particularly, this invention relates to a new and improved "boat" and a method of fabricating the boat so that a plurality of closely spaced wafer-supporting slots can be cut in a pre-assembled boat.
2. Description of Related Art Including Information Disclosed Under .sctn..sctn.1.97-1.99
The assignee of this invention has several patents relating to this general art area. The most relevant patent is U.S. Pat. No. 4,355,974 ('974). Additional patents and publications of relevance to this invention are cited in the '974 patent. In accordance with the cited '974 patent, slots are cut at an angle to the vertical into rods that are thereafter used to form a wafer boat that is used for handling and processing.
Production yield is increased when angle-slotted boats are used and still further increases in production are achievable if the leaning angle of all the wafers is constant throughout the production life of a wafer boat. Such an art area has come to be known as angle-slotted boats to distinguish the Assignee's patented boats and the improved results achievable by the use thereof from the prior art's vertically-slotted boats. The invention of this application, however, is applicable to either type boat or related wafer handling device.
A Japanese patent No. 53-153,861 cited in a Reexamination of the '974 patent and many of the other references cited and discussed during the Reexamination proceedings discloses quite thoroughly the current state of the wafer boat art. In general the side (upper edge) and bottom (lower edge) slots in assignee's angle-slotted boats were formed in rods, or other wafer-supporting material, and the cuts which define the slots consisted of different configurations for the upper and lower slots. Several variations of different slot configurations for the bottom rods are shown in FIGS. 5 through 7 in the '974 patent. An additional prior art bottom slot configuration that has been in use by the assignee and which is not depicted in the '974 patent is in the nature of a small curved depression that is generally elliptically shaped. In accordance with the '974 patent, the side (upper) edges of the wafers are controlled in a desired leaning angle by sloped loosely cut slots and the wafer's weight was supported by a bottom rod which generally positioned the leaning wafers to provide an improved handling and processing operation.
Applicant was the first to purposely make the upper slots wider than the thickness of the wafers to be supported and to divide the wafer-leaning function and the wafer-weight-supporting function into two separate locations, namely the loosely fitting upper and the wafer-positioning lower position. Additionally a pair of opposed upper slots for each wafer are purposely spaced wider apart than the widest expected width of the wafer so that the wafer has room for a side to side movement of the type that it will experience in expanding during the high temperature processing of a wafer batch.
Cuts were first made in the rods and then the boats were assembled to fabricate the wafer boats of the '974 patent. Boat assembly involves high temperature glass fusion and the fusion creates slot alignment or slot deformation problems. It would be preferable if the slotting could be done in pre-assembled boats because the glass fusing of the slotted rods may create such alignment and deformation problems which are caused by high temperatures that are encountered during the assembly process. Slotting of pre-assembled boats, in contrast, avoids these problems.
During use the slotted boats are subject to high semiconductor-processing temperatures and are repeatedly acid washed between furnace firings for cleanliness reasons that are well known to the art. An improved yield can be obtained when the slot angles are maintained at constant angles in spite of repeated acid washings. Most slot configurations do not assure that the coaxially-aligned wafers in the boat will constantly lean at a fixed angle with equal spacing between adjacent wafers after repeated acid washings. Moreover, the slot design problem is further complicated in that adjacent wafer slots must be very closely spaced to keep the production yield at a maximum with lowest possible production costs.
Machine and computer control over X, Y and Z movements of the type necessary to slot angles in a slab or other type of wafer supporting material is well known. A plurality of essentially circular wafers which are coaxially aligned on a common center in a supporting surface, or boat, requires edge support at, or slightly below, the widest diameter position. This support allows use of wafer transfer combs of the type disclosed in the assignee's patent No. 4,568,234 issued Feb. 4, 1986, and now pending in a re-issue Application having Ser. No. 080,437 filed on July 31, 1987.
As noted above, slotting of vertical and angled slots in pre-assembled boats is a known art. The different slot configurations for side (edge) and bottom (weight) wafer contact locations for angle-slotted boats, or tightly fitting slots of the same configuration, has heretofore prevented successful slotting in such boats.
This invention for the first time discloses loose fitting angle slotting of side and bottom wafer locations having the same cross-sectional slot width of about twice the wafer thickness for adjacent wafers in a pre-assembled water boat wherein the leaning wafers touch one wall only at their upper edges. Suffice it to say that highly efficient production has not been achieved through the use of slotted pre-assembled boats prior to the advent of this invention. Moreover, the same unique shape for side and bottom slots which provides a loose fit for supported gravity-fed wafers while still allowing the lean angle of all wafers to be controlled throughout repeated acid washings are key features of this invention.